This isn't so much for the case where the address generation is done by a
simple addition. That could be optimised away by the compiler with an entirely
inline function (as per David Woodhouse's suggestion).
It's far more important for non-x86 platforms which only have a single address
space and have to fold multiple external address spaces into it.
For example, one board I've got doesn't allow you to do a straight
memory-mapped I/O access to your PCI device directly, but have to reposition a
window in the CPU's memory space over part of the PCI memory space first, and
then hold a spinlock whilst you do it.
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